WATCH: Mattis said Tuesday during a press conference in Baghdad that there is a limit on the number of troops he's authorized to deploy in Afghanistan.

WASHINGTON — President Donald Trump's new plan for a more open-ended war against the Taliban, al-Qaeda and ISIS in South Asia was met with cautious optimism by retired Army Maj. Jim Gant, also known as "Lawrence of Afghanistan," one of the few military strategists who had success in the fight in the region.

Gant — a legendary Green Beret counterinsurgency innovator who earned a version of the "Lawrence of Arabia" nickname bestowed on British officer T.E. Lawrence for his famous heroics in the 1916 Arab Revolt — said he was "encouraged" by the Trump plan.

"There are key ingredients for success in this strategy, which bears the clear imprint of Gens. [James] Mattis and [H.R.] McMaster," Gant told ABC News on Tuesday, referring to Trump's secretary of defense and national security adviser.

But the retired Special Forces officer, who was once the target of an assassination order issued by al-Qaeda leader Osama bin Laden because of his success against insurgents in eastern Afghanistan, warned it will "likely take many more years for this conditions-based, Afghan-led campaign to produce any tangible results on the ground — let alone the elusive win that Trump is seeking."

"Few details of how it will be executed were revealed," making it hard to fully assess, Gant said.

The details matter. Troop levels in Afghanistan steadily rose after the 9/11 attacks and the resulting October 2001 U.S. incursion into the Taliban-controlled country by special operations forces — from a few thousand men to more than 100,000 troops at the height of a surge ordered by then-President Barack Obama in 2010.

But adding boots to the war never resulted in tangible strategic gains, as the Taliban — aided by al-Qaeda and Pakistan's intelligence service — also sent a growing stream of jihadis into battle.

The only successful U.S.-led counterinsurgency program was a tribal engagement strategy championed by Gant, a Silver Star recipient and combat-hardened special operator who wrote a 2009 proposal, "One Tribe at a Time," which caught the attention of Navy SEAL Adm. Eric Olson of U.S. Special Operations Command and Army Gen. David Petraeus, who took command of the war in 2010.

Gant's strategy, which when implemented was called Village Stability Operations, had special operations forces living inside Pashtun tribal villages in the most contested rural areas of southern and eastern Afghanistan. They were instructed to befriend the tribesmen — live with them, eat with them, fight with them, bleed with them and even die with them — to win their loyalty and turn them against the Taliban insurgents. Gant led his own VSO team in Kunar province, long al-Qaeda's primary sanctuary, where insurgents called him "Commander Jim." He succeeded in winning over three tribes and thousands of armed fighters there.

"VSO is the only thing bottom up that ever worked," said another retired Green Beret, Lt. Col. Scott Mann, who designed and led the implementation of the countrywide strategy and later authored "Game Changers: Going Local to Defeat Violent Extremism."

Former Special Forces Maj. Jim Gant, sitting second from the right in this August 2011 photo, became close with the Afghan locals he fought alongside against the Taliban.

Despite the program's success in some of the most contested provinces, the Village Stability Operations program was curtailed and phased out after only two years because Obama implemented the Status of Forces Agreement, signed in 2008 by then-President George W. Bush, withdrawing most troops from Afghanistan by 2012.

Gant was relieved of command in 2012 amid concerns about his love affair with then-Washington Post war correspondent Ann Scott Tyson, who is now his wife. As reported in a 2014 ABC News investigation, his commanders claimed he had hidden the fact that Tyson was living with him in a remote village for nearly a year and had stayed out of sight when VIPs visited his outpost. He was forced to accept a demotion and retirement and was even stripped of his Special Forces tab, in spite of his widely heralded success in unconventional warfare. Tyson wrote a book about him, "American Spartan: The Promise, the Mission and the Betrayal of Special Forces Major Jim Gant."

There are almost 10,000 U.S. forces remaining in Afghanistan, including thousands of special operators, but most are restricted from doing anything more than train-and-assist missions with Afghan commandos. Very few counterterrorism missions take place in the country with SEALs and Army Delta Force operators under the United States' Joint Special Operations Command, a senior military official told ABC News.

Mann said Trump's speech left him "underwhelmed" because it amounted to tough talk and few specifics.

"That's when all my spidey-sense started going off," Mann said. "This needs to be a 50- to 100-year campaign. It requires persistence and presence. Colombia should be a model, not Iraq," he added, referring to the Colombian government's five-decade-long conflict with leftist insurgents.

James Sisco, a retired Navy intelligence officer who served as a U.S. military liaison and adviser to then-Afghan President Hamid Karzai, was another champion of the short-lived tribal engagement strategy and is also deeply skeptical that adding U.S. troops again will make a difference.

"Four or five years from now, you'll see no change, and we will still be talking about it," Sisco said. "It will make no difference. If you had robust village stability operations, it would be great. But now you're looking at more guys doing train and assist, not trigger pullers."

Trump said conditions on the ground will dictate strategy, not timetables, adding that he will relax the military's rules of engagement and demand more of NATO partners. Also, he called out Pakistan's government, saying it "gives safe haven to agents of chaos, violence and terror."

His speech bore remarkable similarities to one given by then–presidential candidate Obama a decade ago, who said in the seventh year of America's longest war that "security is most threatened by the al-Qaeda and Taliban sanctuary in the tribal regions of northwest Pakistan. Al-Qaeda terrorists train, travel and maintain global communications in this safe haven."

Little has changed since 2007. Gant said that Trump was right to address Pakistan's duplicity and that his pledge as commander in chief to give ground force commanders more authority to take the fight to the enemy "is encouraging."

"The decision to steer clear of timelines and troop numbers was a smart move, in terms of the kind of psychological warfare that must be conducted aggressively to slow advances made by the Taliban in recent years," Gant said.